Well, I'd say it's probably not your coil or (if using original transistorized ignition) your switchgear as both of those items can work fine when cold but once they are hot they stop working well. I could be wrong.
Did you replace the distributor cap and rotor?
I replaced the cap and rotor a few days ago to no avail. I also changed all HT leads. It has electronic ignition, so there shouldn't be much of an issue there.
The WUR isn't working correctly from the tests I've done (removed filter, no air rushing into it). Probably stuck in the 'hot' position as the car starts up fine and runs alright for around 10 seconds, then begins to idle very poorly and you have to keep it alive with the pedal for another 3-5 minutes. After that, it normally works great.
That brings up the question that I don't believe has yet been answered. What type of fuel management does the car have? Injection like a 280se? Or dual zeniths like a 280s?
The car is the M130 280 SE with fuel injection. I don't think it has any injection issues, other than the WUR.
Here's the thing though. The old man picked it up from the Mercedes dealership after my mechanic fiddled with it (changed the center lead from the coil to the distributor) and fixed a few other things I haven't had time to. It runs perfectly fine again, just drove it around the block. Has great power and I felt absolutely no misfires. I'm hoping that is the end of it, but as stated previously, it does seem to be rather intermittent (humid air issue?).
I'll take it for a drive tomorrow and report back whether it works or not. For the time being, thanks for the help. Here's a picture of the car in question!